Methods and apparatuses for managing fluids in a fuel cell system

ABSTRACT

A fuel cell system including an anode chamber having a fuel mixture comprising methanol and water, and a diffusion layer, a fuel source in fluid communication with the anode chamber via a conduit, a cathode chamber having a cathode and a diffusion layer, wherein the diffusion layer is in fluid communication with an oxidizer, and a proton conducting, electrical non-conducting membrane electrolyte separating the chambers and positioned substantially adjacent to said diffusion layers. The membrane includes a catalyst exposed to each of the chambers for initiating chemical reactions to produce electricity. The system also includes a first valve for automatically controlling a flow of fuel from the fuel supply cartridge, where the first valve includes a shape memory alloy.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/079,733 filed on Feb. 19, 2002, the entire disclosure of which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to fuel cell systems, and moreparticularly, the invention relates to methods and apparatuses forcontrolling fuel flow and sensing fuel concentration in a fuel cellsystem.

2. Background of the Invention

Fuel cells are devices in which an electrochemical reaction is used togenerate electricity. A variety of materials may be suitable for use asa fuel depending upon the materials chosen for the components of thecell and the intended application for which the fuel cell will provideelectric power.

Fuel cell systems may be divided into “reformer based” systems (whichmake up the majority of currently available fuel cells), in which fuelis processed to improve fuel cell system performance before it isintroduced into the fuel cell, and “direct oxidation” systems in whichthe fuel is fed directly into the fuel cell without internal processing.

Because of their ability to provide sustained electrical energy, fuelcells have increasingly been considered as a power source for smallerdevices including consumer electronics such as portable computers andmobile phones. Accordingly, designs for both reformer based and directoxidation fuel cells have been investigated for use in portableelectronic devices. Reformer based systems are not generally considereda viable power source for small devices due to size and technicalcomplexity of present fuel reformers.

Thus, significant research has focused on designing direct oxidationfuel cell systems for small applications, and in particular, directsystems using carbonaceous fuels including but not limited to methanolethanol and aqueous solutions thereof. One example of a direct oxidationfuel cell system is a direct methanol fuel cell system. A directmethanol fuel cell power system is advantageous for providing power forsmaller applications since methanol has a high energy content, thusproviding a compact means of storing energy, can be stored and handledwith relative ease, and because the reactions necessary to generateelectricity occur under ambient conditions.

DMFC power systems are also particularly advantageous since they areenvironmentally friendly. The chemical reaction in a DMFC power systemyields only carbon dioxide and water as by products (in addition to theelectricity produced). Moreover, a constant supply of methanol andoxygen (preferably from ambient air) can continuously generateelectrical energy to maintain a continuous, specific power output. Thus,mobile phones, portable computers, and other portable electronic devicescan be powered for extended periods of time while substantially reducingor eliminating at least some of the environmental hazards and costsassociated with recycling and disposal of alkaline, Ni-MH and Li-Ionbatteries.

The electrochemical reaction in a DMFC power system is a conversion ofmethanol and water to CO₂ and water. More specifically, in a DMFC,methanol, preferably in an aqueous solution, is introduced to the anodeface of a protonically-conductive, electronically non-conductivemembrane in the presence of a catalyst. When the fuel contacts thecatalyst, hydrogen atoms from the fuel are separated from the othercomponents of the fuel molecule. Upon closing of a circuit connecting aflow field plate of the anode chamber to a flow field plate of thecathode chamber through an external electrical load, the protons andelectrons from the hydrogen atoms are separated, resulting in theprotons passing through the membrane electrolyte and the electronstraveling through an external load. The protons and electrons thencombine in the cathode chamber with oxygen producing water. Within theanode chamber, the carbon component of the fuel is converted bycombination with water into CO₂, generating additional protons andelectrons.

The specific electrochemical processes in a DMFC are:

Anode Reaction: CH₃OH+H₂O=CO₂+6H⁺+6e

Cathode Reaction: 3/2O₂+6H⁺+6e⁻ =2H ₂O

Net Reaction: CH₃OH+3/2O₂=CO₂+H₂O

The methanol in a DMFC is preferably used in an aqueous solution toreduce the effect of “methanol crossover”. Methanol crossover is aphenomenon whereby methanol molecules pass from the anode side of themembrane electrolyte, through the membrane electrolyte, to the cathodeside without generating electricity. Heat is also generated when the“crossed over” methanol is oxidized in the cathode chamber. Methanolcrossover occurs because present membrane electrolytes are permeable (tosome degree) to methanol and water. One method of reducing methanolcrossover is to introduce the methanol in an aqueous solution, thusproviding the fuel cell with little more methanol than is required,minimizing crossover without depriving the fuel cell of the necessaryfuel.

One of the problems with using DMFC power systems in portable powerapplications is the lack of a low-cost, effective method for controllingthe concentration of methanol fuel in the fuel mixture. Specifically, aproblem exists in keeping the proper ratio of fuel to water delivered tothe anode chamber in DMFC power systems. For example, if the methanolconcentration on the anode face of the membrane electrolyte is too high,then methanol crossover is likely to occur. Similarly, when a fuel cellis too hot, it may encourage excess methanol crossover. Methanolcrossover not only wastes fuel, and increases the heat of the fuel cell,but can contribute to cathode flooding, which compromises theperformance of the fuel cell.

Because saturation of the cathode prevents the energy producingreactions from proceeding, excess water on the cathode side of themembrane can lead to an increase in methanol concentration at the anode.The increased concentration of methanol may then lead to additionalmethanol crossover resulting in decreased efficiency, a waste ofmethanol, and the generation of unwanted heat.

According, the suitability of DMFC power systems for powering portabledevices and consumer electronics is dependent upon the development ofsystems and methods for controlling the amount of fuel concentration inthe fuel mixture of a direct methanol fuel cell.

Moreover, it is desirable to utilize physical properties of materialsand mechanisms to control the behavior of a fuel cell (i.e., passivecomponents/systems), to reduce product costs and system complexity.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides devices and methods formanaging fuel concentration in a fuel mixture for a fuel cell. Moreparticularly, the present invention provides simple devices and methodsfor controlling fuel concentration which rely on the intrinsicproperties of the materials used to fabricate the fuel cell and fuelcell system. The devices and methods are according to the presentinvention are particularly well suited for use with a direct oxidationfuel cell system.

It is desired to allow physical properties of materials and mechanismsto control the behavior of a fuel cell and/or other associated devicesincorporating fuel flow control while generating heat. By using suchpassive systems, costs can be reduced as well as system complexity. Inaddition, because such a system will have fewer valves and controls,this system will be easier to manufacture on a commercial scale, andwill be more reliable than other more complex systems.

Accordingly, the below recited aspects of the present invention aredirected to direct oxidation fuel cell systems, and more preferably todirect methanol fuel cell power systems.

In one aspect of the present invention, a fuel cell includes a housinghaving an anode chamber in communication with a fuel source via aconduit, a cathode chamber in fluid communication with an oxidizingagent, a proton conducting membrane electrolyte separating the chambers,and a heat-actuated valve proximate the conduit for controlling a flowof fuel from the fuel source to the anode chamber.

In another aspect of the present invention, a method for controllingflow in a fuel cell includes connecting said fuel cell to an electricalload, producing electrical energy, generating heat in response to theproduction of electricity by the fuel cell, and automatically actuatinga heat-sensitive valve for controlling fuel flow into said fuel cellupon the occurrence of a predetermined temperature of the fuel cell as aresult of the heat.

In another aspect of the present invention, a fuel cell includes ahousing having an anode chamber with a fuel mixture, with the anodechamber in communication with a fuel source, a cathode chamber in fluidcommunication with an oxidizing agent, a proton conducting membraneelectrolyte separating the chambers, and a fuel concentration-actuatedvalve for automatically controlling a flow of fuel from the fuel sourceto the anode chamber.

In yet another aspect of the present invention, a method for controllingflow in a fuel cell includes connecting the fuel cell to an electricalload, producing electrical energy, providing fuel to a fuel mixture ofthe fuel cell in response to producing the electricity, and expanding afirst material in response to a fuel concentration of the fuel mixture.The expansion of the first material controls the flow of fuel into thefuel mixture.

In another aspect of the present invention, a sensor for determining aconcentration of fuel in a fuel mixture for a fuel cell includes aconductor that is either in intimate contact with, applied to, ormechanically fastened to a first material. The first material includesan intrinsic property which causes the first material to expand uponexposure to methanol.

In still yet another aspect of the present invention, a method fordetermining a concentration of fuel in a fuel cell includes providing afirst material capable of expanding in response to a concentration offuel in a fuel cell, where within the first material a conductor ispositioned which includes a plurality of individual particle portions.Upon a low concentration of fuel, a substantial portion of adjacentparticles contact one-another. The method also includes passing anelectrical current through the conductor, and measuring the electricalresistance of the conductor. As the fuel concentration is changes,resistance of the conductor changes in proportion (stretching, e.g.,stretching the particles apart) to the fuel concentration. Accordingly,the fuel concentration can be determined based upon the change inresistance of the conductor.

In another aspect of the present invention, a direct methanol fuel cellsystem includes an anode chamber having a fuel mixture comprisingmethanol and water, and a diffusion layer, a fuel supply cartridge influid communication with the anode chamber via a conduit, and a cathodechamber having a cathode and a diffusion layer. The diffusion layer isin fluid communication with an oxidizer. The system also includes aproton conducting, electronically non-conducting membrane electrolyteseparating the chambers and positioned substantially adjacent to, and inintimate contact with each of the diffusion layers, which membraneincludes a catalyst exposed to of the diffusion layers. The systemfurther includes a first valve for automatically controlling a flow offuel from the fuel supply cartridge to the fuel mixture, where the valveincludes a shape memory alloy.

In another aspect of the present invention, a switch for a fuel cell isprovided. The fuel cell includes a housing having an anode chamber incommunication with a fuel source, a cathode chamber in fluidcommunication with an oxidizing agent, and a proton conducting membraneelectrolyte separating the chambers. The switch includes a heat-actuatedshape memory alloy where below a predetermined temperature, the switchis in a first position and upon the fuel cell reaching the predeterminedtemperature, the switch is switched to a second position.

In yet another aspect of the present invention, a switch for a fuel cell(as described above) is provided. The switch includes a first materialhaving expansion properties upon exposure to water, where the switch isin a first position prior to exposure to water and in a second positionafter the first material is exposed to water.

BRIEF DESCRIPTIONS OF THE DRAWINGS

For a better understanding of the invention, reference is made to thedrawings which are incorporated herein by reference and in which:

FIG. 1 illustrates a fuel cell system for which the features of thepresent invention may be aptly used.

FIG. 2 is graph illustrating the loading and unloading behavior ofsuperelastic NiTi alloy.

FIG. 3 illustrates a cross-section of a fuel delivery conduit for a fuelcell system, showing an open conduit and open, heat-actuated valves.

FIG. 4 illustrates a cross-section of a fuel delivery conduit for a fuelcell system, showing a closed conduit and closed, heat actuated valves.

FIG. 5 illustrates a plurality of a heat-actuated valves according tothe present invention.

FIG. 6 is a graph illustrating the expansion/strain of Nafion inresponse to methanol concentration.

FIG. 7A illustrates an overview of a Nafion fuel flow valve for a fuelcell in an open position.

FIG. 7B illustrates an overview of a Nafion fuel flow valve for a fuelcell in a closed position.

FIG. 8 illustrates an electrical fuel concentration sensor using aNafion like material.

FIG. 9 illustrates a visual fuel concentration sensor using a Nafionlike material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, a schematic diagram of one embodiment of a directoxidation fuel cell system 20 includes a membrane electrolyte assembly22 having a proton-conducting, electronically non-conductive membraneelectrolyte 24 disposed between an anode chamber 26 and a cathodechamber 28. The exact shape of the anode chamber and cathode chamber maybe defined by a “flow field channel” which may, but need not be, beintegrated into a flow field plate (not shown), which aids indistributing the fuel and the oxidizing agent to the membraneelectrolyte. In this diagram, each surface of the membrane electrolyte24 is coated with electrocatalysts which serve as anode reactive sites30 on the anode chamber side of the membrane and cathode reactive sites32 on the cathode chamber side of the membrane. Fuel is provided to theanode chamber 26 from a fuel source 46 (e.g., a fuel cartridge) via aconduit 45. The anode and cathode reactive sites facilitate theelectrochemical reactions of the DMFC.

It is worth noting that the electrocatalysts may be provided in otherareas within the anode and cathode chambers, and thus, the invention isnot limited to fuel cells where the catalysts are provided on themembrane electrolyte. Rather the invention is applicable to any fuelcell system where it is desirable to control the flow of fluids withinthe fuel cell system.

Diffusion layers 34 and 36, may be included and positioned on eitherside of the membrane. These layers provide a uniform effective supply ofmethanol solution (diffusion layer 34) to the anode reactive sites and auniform effective supply of oxidizing agent (diffusion layer 36) to thecathode reactive sites. Diffusion layers 34 and 36 on each of the anodeand cathode sides of the membrane electrolyte also assist in providingoptimal humidification of the membrane electrolyte by assisting in thedistribution and removal of water to and from the membrane electrolyteat rates that maintain a proper water balance in the DMFC power system.Moreover, each layer may be used with a flow field (not shown), tofurther aid in distributing fuel and oxidizer to the respective reactivesites.

As previously stated, the anode chamber of the fuel cell for whichfeatures of the present invention may be used also includes the flowfield plate (not shown) which also functions as a conductor (i.e., actsas the electrical anode), and an exhaust vent 38 which allows carbondioxide created during oxidation of the fuel to pass out of the anodechamber. Similarly, the cathode chamber may include a flow field plate(not shown) which guides oxidizing agent in the chamber and alsofunctions as a conductor (i.e., acts as the electrical cathode), aninlet 40 and an exhaust outlet 42 which allows air to flow through thecathode chamber so that an adequate supply of oxygen is insured for thereaction. One skilled in the art will appreciate that air may flow frominlet 40 to outlet 42 and in the opposite direction, when the system isexposed to an ambient air pressure.

In a DMFC power system, an aqueous methanol solution, preferably asolution greater than 0% to about 100% methanol stoichiometrically, andmore preferably between greater than 0% to about 50% methanolstoichiometrically is introduced as the carbonaceous fuel reactant. Themethanol solution circulates past the anode reactive sites 30. Upon theapplication of an electrical load between the flow field plates of theanode and the cathode chambers, the methanol solution disassociates,producing hydrogen protons and electrons, and generating carbon dioxideas a first by-product of fuel oxidation. Hydrogen protons migratethrough the membrane electrolyte to the cathode chamber while electronspass through the external load. The protons and electrons then combinewith oxygen in the cathode chamber to form water, the second by-productof the reaction. The electrons are retrieved by the flow field plate ofthe anode chamber and carried through an external electrical load 44 tothe flow field plate of the cathode chamber.

First Embodiment

In a first embodiment of the present invention, flows of fuel, gases,liquids and effluents is controlled by a heat-sensitive switch/valve. Aparticular aspect of the present invention is directed to controllingthe flow of fuel from the fuel source to a mixing chamber, an anodechamber or to a pump. With this embodiment, as the operating temperatureof the fuel cell increases, the flow of fuel is either increasinglyrestricted by the switch/valve or quickly restricted. Ultimately, uponthe fuel cell reaching a predetermined operating temperature, the flowof fuel is completely shut-off by the novel switch/valve.

The first embodiment is realized via use of bi-metals and shape memoryalloys used as springs. Bi-metal springs are springs where two metalsare bonded together which have different thermal strain rates. As theby-metal spring is heated, the strip bends allowing it to perform work.Shape memory alloys, on the other hand, undergo a phase transformationin their crystal structure when cooled from a stiffer, high temperatureform (Austenite) to a more malleable, low temperature form (Martensite),giving the alloys shape memory and superelasticity.

When a shape memory alloy is in its martensitic form, it is easilydeformed to a new shape. However, when the alloy is heated through itstransformation temperatures, it reverts to austenite and recovers itsprevious shape with great force. This process is known as shape memory.

The temperature at which the shape memory alloy returns to its hightemperature form when heated can be adjusted by slight changes in alloycomposition and through heat treatment. In NiTi (nickel-titanium, e.g.,Nitanol) alloys, for example, the temperature can be changed from above+100 deg. C. to below −100 deg. C. The shape recovery process occursover a range of just a few degrees and the start or finish of thetransformation can be controlled to within a degree or two if necessary.

Supereleastic behavior can be accomplished if the alloy is deformed at atemperature which is slightly above their transformation temperatures.This effect (shown in graphical form in FIG. 2) is caused by thestress-induced formation of some martensite above its normaltemperature. Because it has been formed above its normal temperature,the martinsite reverts immediately to undeformed austenite as soon asthe stress is removed.

This process provides a very springy, rubber-like elasticity in thesealloys, and can create sufficient force to open or close a valve.

Accordingly, the first embodiment of the present invention uses theabove materials to create a heat sensitive switch/valve for controllingthe flow of fuel; preferably, such materials are shape memory alloys,with a nickel titanium alloy being most preferable. As shown in FIGS. 3and 4, pieces of shape memory alloy (SMA) are placed adjacent a flexiblefuel flow tube which is positioned proximate a portion of the fuel cellbody which adequately reflects the temperature of the fuel cell (shapememory alloy springs are illustrated, but bi-metal springs include asimilar arrangement).

When the fuel cell is connected to an electrical load, and electricityis produced, the fuel cell begins to heat up as a result of the chemicalreaction of methanol with the catalysts contained within the fuel cell.As such, the threshold temperature can be calibrated such that the SMAis activated if a certain temperature is exceeded, thus adjusting theflow of reactants to prevent the fuel cell system from overheating. Whenthe fuel cell becomes too hot, may indicate a high fuel crossover rate,or excessive demand on the fuel cell Each of which may excessivelystress the system, which may result in an increased in liquid buildup onthe cathode, or CO2 buildup on the anode, among other problems, leadingultimately to a slowing reaction and lowered electricity production.Where multiple inventive valves are implemented within a single system,they may be configured to transform at different temperatures, thusproviding different responses under a variety of conditions. The SMAspring is placed at desired locations adjacent the flexible fuel flowtube and proximate a portion of the fuel cell having a temperaturecorresponding to the operational temperature of the fuel cell. When thetemperature of the fuel cell portion reaches a predeterminedtemperature, the SMA spring reverts to its austenite shape, where thecenter of the spring bends inward toward the center of the flexible flowtube, thereby immediately closing the flexible tube in on itself andcutting off the fuel supply. Of course, the SMA spring is preferablyshaped and designed such that the interference of the fuel flow occursonly when a predetermined first temperature of the fuel cell is reached.

Depending upon the design of the fuel cell, the actuation of the SMAspring can be custom tailored for temperatures as well as to the effecton the supply of fuel. Specifically, the spring may be designed suchthat upon activation, fuel is slowed to a predetermined point, or iscompletely shut off.

Accordingly, as shown in FIGS. 3 and 4, SMA springs 50 include a firstend 52 positioned against a conduit wall 56, and a second end 54positioned against conduit wall 56. The SMA spring may include a slightvisible curve 58, alluding to the direction upon which the SMA springwill bend, however, this curve may be so slight that it is not whollyvisible to the naked eye.

Although the present invention is shown and described using two SMAsprings, a single SMA spring may be easily designed to perform theintended valve/switch function. The SMA springs can be shaped andprogrammed to pinch a flexible tube at a certain cell temperature,thereby restricting the liquid flow totally or in part. Moreover, thespring shape illustrated need not be in the shape of a leaf spring. TheSMA spring may be shaped as a coil spring which crimps the flexible tubeon its own without pushing against an object.

Alternatively, a spring may allow the introduction of water or a moredilute mixture if a certain temperature is reached, by placing a springthat, in its Martensite configuration prevents the flow of a fluid, butwhich when its austenite state is attained, it allows the flow of adesired fluid.

As stated earlier, bi-metal springs (BM) may also be used to implementthis characteristic. BM springs have the effect of increasinglyinterfering with the affected flow since the characteristic of theirbending is that they continually bend upon increasing temperature. Incontrast, SMA springs remain in one shape, then revert back to anaustenite shape upon a predetermined temperature. This idea of therestriction of fuel flow with a BM spring valve based on temperature canbe best understood with reference to FIG. 5. As shown, temperatures T4is greater than temperature T3, which is greater than temperature T2,which is greater than temperature T1. These temperatures represent, forexample, initial startup temperature (T1), a temperature a firstpredetermined time after startup in which fuel flow is starting to berestricted (T2), a temperature after a second predetermined time afterstartup in which fuel flow is increasingly restricted (T3), and atemperature after the fuel cell has reaches the threshold temperature inwhich fuel is effectively shut-off (T4).

Accordingly, for both SMA springs and BM springs, when the operatingtemperature of the fuel cell decreases because of fuel restriction bythe spring valve or via switching off the electrical load, either thespring completely retracts (SMA springs), or the spring retracts to aposition corresponding to the temperature (BM) spring.

Second Embodiment

The second embodiment of the present invention is directed to the use ofa material which expands upon exposure of methanol. More particularly,the second embodiment is directed to a material whose expansion isdirectly related to a concentration of methanol fuel in a fuel solution.Such a feature may be used as a switch, valve or sensor in a fuel cell.

A material having these qualities is Nafion® 117, manufactured by E.I.du Pont de Nemours and Company of Wilmington, Del.,. As shown in FIG. 6,which is a graph of size/strain created versus percentage of methanol insolution, Nafion includes a first size in a dry state, a somewhatexpanded form when exposed to water, and then a proportional sizedepending upon a percentage of methanol in solution. The amount whichNafion 117 expands is predictable and essentially linear over therelevant methanol concentrations. As such, the expansion of Nafion canbe used to actuate a switch or a valve to control a flow in a fuel cell.

For example, as shown in FIGS. 7A and 7B, a piece of Nafion likematerial 60 may be used as a valve actuator in a fuel cartridge 62 orconduit. As shown, upon a low concentration of methanol, the Nafionremains at a first size in FIG. 7A where fuel is allowed to freely flowfrom the cartridge to be ultimately used in the anode chamber. However,the Nafion member begins to expand as the level of methanol increases.Once the concentration reaches a predetermined percentage, the Nafionhas expanded such that it now closes a valve supplying fuel to thereaction. Those skilled in the art will recognize that there arenumerous ways in which a switch could be mechanically actuated.

As used as a sensor, Nafion can easily communicate a concentration levelof methanol. Accordingly, as shown in FIG. 8, a conductor 70 (having,for example, particle portions 70 a) is applied to or mechanicallyfastened to a piece of Nafion or other similar material 72. In thisembodiment, the conductor is laid on such material in a serpentinepattern (for example) and has a current passing through it. Theconductor, also, has a known resistance value under certain states ofstrain and a known set resistance value, where the total resistancevalue of the conductor as a whole changes depending upon the degree towhich the conducting particles are in contact with each other. Thus, ifthe conductor is in a relaxed, rather than a strained state, theconductive particles are largely in contact with one another, and thetotal resistance value of the conductor is a first value.

Upon exposure of the Nafion/conductor methanol concentration sensor, theNafion expands. As the Nafion expands, the contact between theconductive particles of the conductor is diminished, and thus theresistance of the conductor increases proportionally. Thus, theinventors have found that the change in resistance of the conductorwithin the Nafion based upon the present invention is directlyproportional to the concentration of methanol in a methanol solution.

Such a system can be easily fabricated and integrated into a fuel cellto directly monitor fuel concentration. With regard to monitoring theresistance of the conductor, a simple Wheatstone bridge circuit may beused to determine the actual resistance. Thus, the resistance values arecompared to known values associated with particular fuel concentrations.

Other materials with identified properties may also be used as a visualsensor as shown in FIG. 9. There, Nafion like material is displayed in awindow element to visually gauge fuel concentration to a written scalelocated adjacent the window. Specifically, a conduit 80 having the fuelmixture of the anode (or direct access thereto) includes window 82,having a written legend 84 comprising fuel concentration values (e.g.,0-4%). A Nafion like material 86, having one end fastened to a anchor 88of the conduit is placed adjacent the window. Thus, with increasing fuelconcentration, the Nafion like material expands. The top end is thencompared to the visible scale to determine the fuel concentration.

The embodiments described above may be used not only as a valve, butalso as a sensor and a switch for any associated switching applicationin a fuel cell. Such applications include valves for fuel flow, airflow, flushing effluents and the like.

Having described the invention with reference to the presently preferredembodiments, it should be understood that numerous changes in creatingand operating such systems and methods may be introduced withoutdeparting from the true spirit of the invention as defined in theappended claims.

1-88. (canceled)
 89. A method for controlling flow in a fuel cell,comprising: producing electrical energy in the fuel cell; and actuatinga thermally-sensitive actuator based on a temperature of the fuel cellfor controlling a flow.
 90. The method according to claim 89, whereinsaid thermally-sensitive actuator increases or decreases said flow. 91.The method according to claim 89, wherein said flow comprises a flow offuel to the fuel cell or a flow of water to the fuel cell.
 92. Themethod according to claim 89, wherein said actuator comprises a shapememory material, alloy and/or a bimetal material.
 93. The methodaccording to claim 92, wherein said bimetal material comprises a nickeland/or titanium alloy.
 94. The method according to claim 89, whereinsaid thermally-sensitive actuator is actuated in response to heatgenerated by the fuel cell.
 95. A method for controlling a flow in afuel cell, comprising: producing electrical energy in said fuel cell;providing a flow of a fluid to a fuel mixture of said fuel cell inresponse to said production of electrical energy; and expanding a firstmaterial in response to a fuel concentration of said fuel mixture,wherein expansion of said first material controls said flow.
 96. Themethod according to claim 95, wherein said flow comprises a flow ofwater or a flow of fuel.
 97. The method according to claim 95, whereinsaid first material comprises Nafion.
 98. The method according to claim95, wherein said expansion of said first material increases or decreasessaid flow.
 99. A method for determining a concentration of fuel in afuel cell comprising: providing a dimensionally variable first materialcapable of expansion and contraction in relation to a concentration offuel in a fuel cell, wherein a conductor is disposed on or within thefirst material; flowing an electrical current through said conductor;measuring an electrical property of said conductor, wherein as fuelconcentration changes, the first material expands resulting in aproportionate change to the electrical property of said conductor. 100.The method according to claim 99, wherein the electrical propertycomprises at least one of resistance, impedance, and conductance.